//Find the missing letter in the passed letter range and return it.If all letters are present in the range, return undefined.
function missingLetter( str ) {
  for ( let i = 1; i <= str.length; i++ ) {
    if ( ( str.charCodeAt( i ) - str.charCodeAt( i - 1 ) ) > 1 ) {
      return String.fromCharCode( str.charCodeAt( i - 1 ) + 1 )
    }
  }
}
console.log( missingLetter( "abcdefghjklmno" ) );
console.log( missingLetter( "abce" ) );
console.log( missingLetter( "abcdefg" ) );
//("acd").charCodeAt(0)-> 97
//String.fromCharCode(("acd").charCodeAt(0)) -> "a"
